RELATIVE REACTIVITY AND STEREOSELECTIVITY IN THE WITTIG REACTIONS OF SUBSTITUTED BENZALDEHYDES WITH BENZYLIDENETRIPHENYLPHOSPHORANE

Carbonyl carbon-14 kinetic isotope effects and substituent effects on the relative reactivity and on the cis-trans product ratio were determined in the Wittig reaction Of XC6H4CHO with Ph3P=CHC6H4Y in THF at 0-degrees-C. The isotope effect and the Hammett rho-value were positive under both Li salt-free (12k/14k = 1.060 +/- 0.003 and rho(X) = 2.77 +/- 0.15) and Li salt-present (12k/14k = 1.015 +/- 0.004 and rho(X) = 1.38 +/- 0.12) conditions, although they were much larger in the former case. These, together with the absence of enone isomerization for the benzylidene ylide reported previously, suggested that the reactions proceed via a polar cycloaddition transition state of considerable nucleophilic character. The cis-trans ratio of the product stilbene was essentially unchanged (40:60 in the Li salt-free and 60:40 in the Li salt-present reaction) by the change in concentration, the mode of addition, and the molar ratio of aldehyde and ylide, and it was varied only slightly for most substituents X and Y. However, the ratio was significantly varied when o-MeO or o-Cl was introduced as X. The results could be rationalized by assuming a chelating interaction between the lone pair of the ortho substituents and the phosphorus of the ylide.